Method for producing particle-shaped material

ABSTRACT

A method for producing particle shaped material from a melt. The method comprises the following steps: the material is melted, the material melt is intimately mixed with a gas in a predetermined ratio; the gas-liquid mixture is suddenly expanded and the material powder thus produced is collected, which the other gas is added to according to a predetermined air/liquid ratio during the expansion with solidification of the formed material drop, to form a material powder having a controlled size distribution and shape in an area of lower pressure than the gas/liquid mixture.

BACKGROUND OF THE INVENTION

The Invention pertains to a method for controlled production ofparticle-shaped material from the molten substance.

Production of particle-shaped material from molten substance is knownfor plastic, natural substances, glasses and even metals. There is aconstantly increasing demand for powders of such types of materialshaving small grain size.

These known methods produce a particle-shaped material which can also bereferred to as powder, granulate, smalls or something similar. For thesake of simplicity, these particle-shaped materials are referred tobelow as powder. These kinds of powders are used for example ininjection moulding techniques, production of alloys, sinteringtechniques, bonding materials, catalysts, paints and lacquers, cellularplastic production etc. The market for these applications has a highdemand of powder having small. grain size of certain particle shapes inlarge quantities at low cost. Till now these types of powders wereproduced a.o., in that—as described for example in the document WO01/62987, fluid fused substances like oxidic slag, glasses or fusedmetal are nozzle jetted or atomised in some other way in a chamber byhigh pressure gas expansion—for example, with the help of rotatingplates/discs which centrifuge away the generated droplets and thuspulverise (Rotating Disc Method) , or even by means of RollerAtomisation—whereby molten metal droplets meet an rotating rollers andare centrifuged away from them and get solidified an the flight path.Another typical method is water atomisation. To a large extent however,mainly gas atomisation is used. For this, nozzles, like Laval-nozzlesare used, in which the fluid molten substance is first stronglyaccelerated and thereafter sporadically expanded under strongacceleration to a high velocity at the nozzle exit into a chamber andthus atomised.

A typical method is described in the Austrian patent document 2987350 ofEckart-Werke. Ideally, the molten substance is further distributed bygas currents simultaneously along with this sporadic expansion at thenozzle outlet and thus finely distributed fused droplets are obtainedwhich get solidified an cooling and thus form the powder. Hence theknown generic method consists of bringing a material melt after exit outof the nozzle in contact with gas and simultaneously sporadicallyexpanding it. This known method is disadvantageous to the extent that itwas very difficult to control, had to be interrupted very frequently andcould not be carried out continuously. Finally, the energy costs werealso very high.

From the document WO 99/11407 of Pacific Metals Co. Ltd. it is knownthat one can obtain metallic powder through atomisation of fused-fluidmetal, in that an away current of the fused-fluid metal is atomised atthe exit of a nozzle by introducing it into the centre of the nozzlealong with a laminar-gas current enveloping it and by introducing afluid current an an exit of both the currents from a conical nozzle,which strengthen the atomisation. In this case, in addition to the gascurrent a fluid current should also be provided, which is complicated.

From the document EP-A-1038976 a method is known, in which sprayedmolten droplets in the inner region of a spray jet are heated up onceagain in the inner region of a cooled cooling chamber by means ofpost-combustion of hot gases after exit from the spray nozzle and thus abetter distribution of the molten droplets is achieved by means of theheated combusting gases and hence the fineness of the produced powder isimproved. This method is very energy intensive, as it requires coolingof the walls of the cooling chamber and also cumbersome measures willhave to be carried out for producing small particle sizes.

A disadvantage of the known method is also, that these did not try tocontrol the particle size or shape; a uniform particle size spectrum wasgenerated by setting the gas/fluid-ratio, on which no other influencecould be exercised.

It is therefore the task of this invention to create a continuous,efficient method for controlled production of powders from materialmelts, which would allow control of size and shape of the producedparticles.

SUMMARY OF THE INVENTION

This task is fulfilled by the invention with the help of a method forproducing particle-shaped material from molten substance with melting ofthe material, inner mixing of the molten material with a gas in apre-defined ratio; sporadic expanding of the gas/liquid mixture; andcollection of the thus produced material powder, in which addition offurther gas in a pre-defined air/liquid ratio during the expansion undersolidification of the formed material droplets to the material powder ina controlled size distribution and shape into a chamber having lowerpressure than the gas/liquid mixture.

It was surprisingly found out that with the help of the double settingof the gas/melt ratio as per the invention, in the first mixing stepwithin the melt and with the sporadic expansion at the nozzle exit, theparticle size spectrum is controllable with respect to the averageparticle size as well as with respect to the total particle sizedistribution spectrum, whereby with the consumption of the same amountof gas smaller particles can be attained than in the case of methodswhich feed gas only once.

In the first mixing step, an addition of more gas to the melt, theaverage particle size gets reduced.

The ratio of the mass flows of both phases GLR=m_(gas=)mli_(q)(Gas/Liquid-Ratio) is determined by the following factors:

-   -   1)The inlet cross sections into the mixing chamber A_(gas) or        ALiq    -   2) The preliminary pressures P_(0.gas) or P_(0.liq) at the        inlet.

According to the Invention, both these factors can be regulated asdesired, in order to ensure a constant ratio GRL during nozzle jetting.The influence of the GLR an the powder fineness is shown in FIG. 4. Fromthat one can see that by regulating the inlet cross sections A_(gas) orA_(liq) or with the help of the available preliminary pressuresB_(0.gas) or B_(0.liq), both of which conform to the mass ratio of bothcomponents, the particle fineness can be controlled.

Addition of further gases at the nozzle exit leads to even finermaterial. For this, it is important to know the flow ratios in themixing chamber: depending an the empty pipe velocity J of the fluid andgaseous phase (VLR.1=M_(i/A)), different flow regimes could get formed(FIG. 6); in the method as per the invention one works with blow flow orring-(spray-) flow. The empty pipe velocity for gas J_(G) and liquidJ_(L) can be obtained by the experts from the diagram shown in FIG. 7.

Blow Flow

While working in the region of blow flow in the mixing chamber the gasphase does not form any bubbles which expand an account of drop inpressure and then burst at the point of maximum pressure drop (in theregion of drop exit). The continuous liquid phase is thereby dividedinto ligaments which disintegrate into droplets in a second step. Thecritical parameter for this secondary droplet disintegration is therelative WEBER-NUMBER We_(rel)(see Wallis, G., B: (1969)—One-DimensionalTwo-Phase Flow: New York: Mcgraw-Hill)We_(rel)=(ρ_(gas) ·d _(drop.V) ²rel)/σ

-   For V_(rel)=V_(gas)−V_(liq) relative velocity between gas and liquid    (m/sec)-   D_(drop)=diameter of the liquid drop (m) (ρ_(gas)=density of the    gas. (kg/m³)-   σ=surface tension of the melt (N/m)-   For secondary drop disintegration We_(rel)<12-13 holds good

In order to achieve secondary drop disintegration with a water drop sizeof say d_(drop)=50 μm at the nozzle exit, a relative velocity would berequired between the drop and the surrounding gas of around 125 m/s,which however can never be attained an account of the limiting twophasesound velocity. Now by bringing up an outer mixing one surprisingly getsthe required velocity, in order to atomise the ligaments and dropspresent at the nozzle exit in an energyefficient manner. In this way,with the gas velocity of the outer mixing, the particle diameter of thepowder can be controlled. The above mentioned relation between thedimensionless Weber number holds good for all liquids (even metals).This means that with the help of this method one can also process Cu,Al, Zn, Sn etc. to powder.

In the following tables the critical velocities are given, which arenecessary for a secondary atomisation for a 50 μm drop of variousmetals:

Surface tension a Critical velocity v (N/m) (m/s) Water 0.07 125 Copper1.16 500 Tin 0.68 380 Aluminium (99%) 0.18 196 Lead 0.43 303 Iron 1.8620 Silver 0.91 440

In case of atomisation of metals and other materials the followingshould further be taken into account: the time required for secondarydrop atomisation is often longer for materials than the time which thedrop takes for solidification. Also because of this, one cannot achievesuch a good result in the drop (=powder) fineness with a purely internalmixing.

Ring Flow

With increasing gas flows in the mixing chamber the bubbles coalesce anda ring-shaped liquid film gets formed along the chamber walls, the gasflows in the core of the chamber and rips with increasing velocity (forexample, in case of pressure drops) droplets out of the liquid film.

The liquid film present at the nozzle exit (the disintegration mechanismdescribed for blow flow also occurs analogously in case of ring flow)are again subjected here to a secondary air fed from outside, whichyields the required good atomising result, as one can see in FIG. 8.

The particle shape can also be similarly controlled by the method as perthe invention; thus round particles are obtained for inert gas, whereasoblong particles occur particularly in case of hot gas (air). Theparticle shape can be of extreme importance for the application of theproduced powder and is therefore a significant parameter.

Due to the fact that initially a defined inner melt/gas mixing isproduced and this is then expanded alter producing this mixture bysporadically adding further gas, one surprisingly obtains a bettercrushing of the material droplets, a control of the particle shape aswell as an adjustable particle size spectrum. It has also beensurprisingly seen that the energy consumption of this method can besignificantly reduced as compared to known gas atomisation methods whichhave to use significantly more gas at significantly higher pressure.

The inner mixture of melt/gas from an atomising unit with a velocityincrease of the liquid droplets expands to 30-100 times the conveyingvelocity from the melting container into a receiving container. The gasparticles get accelerated even more strongly an account of their lowmass and account for expansion of the exiting jet resulting in improvedatomisation. For improving the atomisation, the expansion can be carriedout by nozzle jetting the inner material mixture into a chamber havinglower pressure. The chamber can also be cooled in order to acceleratethe cooling.

For fused metals it could be particularly advantageous to set the innermaterial of the gas mixture to a ratio gas flow/molten material of0.05-15, preferably 0.05-3 and more ideally 0.3-1.5 kg-/kg of moltenmaterial.

As material, for example, meltable plastic offers itself—plasticgranulates are required for injection moulding machines etc.Furthermore, nozzle jettable material say from recycling material can beconsidered and the thus produced granulates can be further used in avery simple manner—e.g. as additive for cement or similar items. Othersuitable materials are metals like Zn, Ni, Al, Ag, Ng, Si, Ca, Cu, —Ni,Mo, Pb, Ti, Sn, Li, Be, W, Fe, Co, Cr, Mn, Be and especially theiralloys. Metals in powder form are desired for the most varied forapplications, e.g. for metal casting, like injection moulding, forBonding materials, catalysts, paints, colours.

For all these applications it is apparent that also the particle shapeplays an important role; thus round particles have a different showerbehaviour and angular friction and are better suited for completefilling up of moulds without hollow spaces and can more easily beconveyed an account of lower inner friction than oblong, spurtingparticles which have greater friction against one another, which howeveris desirable for different application forms, e.g. for producingmouldings or for extrusion moulding.

According to the method as per the invention the most varied melts, likeglasses and meltable ceramics, metals can be nozzle jetted. However,even meltable natural substances like grease or wax which are solid atsurrounding temperature can be processed.

As gases one can use gases known as suitable to the expert for thematerial to be nozzle-jetted in case oxidation plays only a subordinaterole, the gas could be precious air—one could however also work withinert gas which does not react to a noteworthy extent with the materialto be nozzle jetted—e. g. nitrogen or argon. Ideally the gas is selectedfrom the group consisting of inert gases, like inert gas, helium, argon,nitrogen—which also influence the particle shape—or gases partiallyreacting with the melt like nitrogen, air, carbon dioxide, carbonmonoxide, water vapour, combustion gas or even mixtures of the saure,whereby the classification of the gas as inert gas depends an the natureof the material to be nozzle jetted, as is known to the expert. Thecombustion gases include especially also combustion gases produced insitu. By combustion gas generated in situ one refers to such a gas whichis formed in the melt from the combustible components—e.g. by additionof hydrocarbons to the melt which also have oxidation-inducingcomponents—e.g. oxygen-containing components.

Due to combustion the gas volume suddenly increases manifold and themixture of gas/melt get swirled through.

In a typical application of the method, the melt particles of the innermixture is accelerated to 5-100 times the velocity during anexplosion-pipe expansion at the nozzle exit, whereby they getsolidified. A typical particle velocity in the mixture which is conveyedin a pipe to the mixing chamber is 0.1 m/sec. and is then accelerated to50-100 m/sec. during nozzle-jetting. The gas particles of the mixtureget accelerated significantly higher an account of their lower mass,somewhat to the magnitude of 1000-times.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objectives, features and advantages can be obtained from thefollowing description and the claims along with the accompanyingdrawings. The following are shown:

FIG. 1 A schematic representation of the steps of the method;

FIG. 2 a perspective part-view of an arrangement as per the inventionwhich can be used for conducting the method;

FIG. 3 a sectional view of a nozzle to be used in the method as per theinvention;

FIG. 4 the result of nozzle jetting of aluminium/air mixture withoutadditional feeding of the air at the nozzle exit;

FIG. 5 the result of nozzle jetting of aluminium/air mixture withadditional air feeding at the nozzle exit;

FIG. 6 schematic representation of flow profiles;

FIG. 7 flow pattern as a relation of the empty pipe velocity of gas andliquid in the mixing chamber;

FIG. 8 comparison of nozzle jetting of aluminium with air with andwithout feeding of air at the nozzle exit—with the absolute quantity ofair fed being the same.

DETAILED DESCRIPTION

Preferred design versions are described below—this is however notrestricted in any way to this application—with the help of this method,similarly other meltable material, other metals e.g. nickel, tin,silicon, titanium, metal alloys like bronze; glass or even glasses,meltable plastics (thermoplastics), natural substances like grease andwax as well as other materials can be pulverised.

In FIG. 1 the process sequence according to the principle of theinvention is schematically depicted. As shown, the method includes theproduction of a melt by melting at increased temperature; mixing of themelt (in case of metals) in a ratio of gas/metal of approx. 0.05-15kg/kg. The mixing should be internal as far as possible, so that the gasis uniformly mixed in larger quantity below the melt. After the mixingstep the thus produced gas/liquid mixture is sporadically expanded by anozzle and sent into a region having lower pressure, whereby thematerial droplets get solidified and become particles with particle sizeof <1000 μm, which get solidified during flight and collect in thecollecting container. Simultaneously with the sporadic expansion, gas isonce again fed at the nozzle exit, whereby an even stronger expansion ofthe material droplet current results; these are distributed in a finermanner and a controlled/regulated finer particle size spectrum isproduced. This depends to a large extent an the type of material thusatomised and the gas used, as has been described in details above.

In FIG. 2 a diagram of a portion of a plant for conducting the method asper invention is shown. These types of plants are known to the expert inall important aspects. A typical plant is, for example, the onedescribed in the document DE-A-2007 803. The known plants are nowextended by the invention in such a way that a mixing unit is used forproducing the inner gas/material mixture before the nozzle. Such mixingunits for gas/melt mixtures are known to the expert and can be selectedby him conforming to the melts to be processed, like electromagneticstirring, finely distributed introduction of gas, like through frittingetc.

A typical ratio of gas/material in the inner mixture should lie—e.g. foraluminium—in the range of 10-60% by weight. On using inert gas one coulduse lesser gas, in the region of 10-30% by weight; in case of air andgas the ratio lies between 20-60% by weight of gas in the mixture.

By varying the gas content the particle size distribution can bepostponed here—the average value of the particle size varies.Furthermore, significantly lesser air needs to be used than in the gasatomisation method according to the state-of-the-art technology. Now itis possible, instead of a gas/metal ratio of 10:1 in thestate-of-the-art technology which feeds gas only once, to work with aratio of 0.5-1.5 kg/kg of gas/metal, which means a significant saving,as only approx. 1/10 of the gas quantity has to be fed in order toobtain comparable average particle sizes; furthermore, the methodsaccording to the state-of-the-art technology do not allow forcontrol/regulation of the particle shape or the particle size spectrum.

A typical nozzle used in such types of applications for sporadicexpansion is shown in FIG. 3. Here one can clearly identify theintroduction of gas in the exit region of the gas/material mixture,which leads to a better guiding of the current of the exiting materialand significantly reduces a fixation of the solidified material an thenozzle and reduces the particle size.

One can thus surprisingly see that by using a gas/material melt mixture,the addition of gas while using nozzles with gas entry at the exit ofthe melt can be significantly reduced for the same yield of materialpowder and the particle size spectrum as well as the particle shape canbe controlled.

Nozzle-Jetting of Zinc.

Zinc is melted at a temperature of approx. 500° C. (melting point: 420°C.). The liquid metal is mixed with air in a mixing chamber in a ratioof 1 Kg air/Kg Zn and then nozzle jetted by a Laval-nozzle connected tothe mixing chamber, whereby air is introduced once again at the nozzleexit in a ratio of approx. 0.5 kg/kg. Powder with an average grain sizeof d₅₀=70 μm and a particle size between 3 and 200 μm was obtained. Theparticles had an oblong,-spurting shape.

Nozzle-Jetting of a Zinc-Copper-Alloy

A zinc-copper alloy is melted at a temperature of approx. 800° C. Theliquid metal is mixed with air in a mixing chamber in a ratio of 1 Kgair/kg zinc-copper alloy and then nozzle-jetted through a Laval-nozzleconnected to the mixing chamber, whereby air is once again introduced atthe nozzle exit in a ratio of approx. 0.5 Kg/Kg. Powder with an averageof grain size d₅₀: 60 μm and particle size between 3 and 200 μm wasobtained. The particles had an oblong,-spurting shape.

Nozzle-Jetting of Copper

Copper is melted at a temperature of approx. 1220° C. The liquid metalis mixed with air in a mixing chamber in a ratio of 2 Kg air/Kg Cu andthen nozzle jetted through a Laval-nozzle connected to the mixingchamber, whereby air is once again introduced at the nozzle exit in aratio of 1.5 Kg/Kg Cu. Powder with an average grain size of d₅₀: 76 μmand a particle size between 3 and 200 μm was obtained. The particleswere almost round.

Nozzle-Jetting of Aluminium

Aluminium is melted at a temperature of approx. 700° C. The liquid metalis mixed with air in a ratio of 0.4 Kg air/1 Kg Al in a mixing chamberand then nozzle jetted through a Laval-nozzle, whereby during nozzlejetting 0.4 Kg air/Kg A1 was added. Powder having an average grain sized₅₀: 45 μm was obtained, whereby the grain shape was oblong to round.

On using a ratio of 2 Kg of air/Kg Al an average grain size d₅₀=28. μmwas obtained.

Leaving out the addition of gas at the nozzle-exit led to a changedgrain size average (see FIG. 4 and 5) for the same quantity ofgas/melted metal. One can clearly identify the favourable influence ofexternal air feeding an the grain size in FIG. 5. With increasing airfeeding the grain size drops, whereby this influence an application ofexternal air for the same air/metal ratio results in significantlysmaller particles. As one can see from FIG. 8, for the same quantity ofair fed the addition of air in a mixing chamber and subsequently at thenozzle exit results in significantly smaller particle size than feedingthe same air mixture in the mixing chamber alone; here the averageparticle size gets reduced by half.

Nozzle-Jetting of Magnesium

Magnesium is melted at a temperature of approx. 700° C. under nitrogenatmosphere. The liquid metal is mixed with nitrogen in a ratio of 1 KgN₂/1 Kg Mg in a mixing chamber and then nozzle jetted through aLaval-nozzle, whereby during nozzle jetting 0.5. Kg N₂/Kg Mg was added.Powder with an average of d₅₀: 70 μm was obtained, whereby the grainshape was oblong to round. On using a ratio of 0.2 kg N₂/Mg an averagegrain size of d₅₀ =54. μm was obtained. Leaving out the addition ofnitrogen to the mixing chamber resulted in larger particles with d₅₀=180μm for the same gas quantity/Mg. Leaving out the gas addition at thenozzle exit led to a size of d₅₀=120 μm. From this one can conclude thatonly double gas addition delivers the desired effect.

Nozzle-Jetting of Steel

Steel is melted at approx. 1550° C. The liquid metal is mixed withnitrogen in a mixing chamber at a ratio of 1 Kg N₂/1 Kg of steel andthen nozzle jetted through a Laval-nozzle, whereby during nozzle jettingonce again 0.5 Kg N₂/Kg of steel was added. Powder with an average ofgrain size of d₅₀:80 μm was obtained, whereby the grain shape was oblongto round. On using a ratio of approx. 2 Kg N₂/Kg of steel an averagegrain size of d₅₀=62 μm was obtained. Leaving out the addition of gas tothe mixing chamber led to powder with d₅₀=221 μm for the same gasquantity/molten metal. Leaving out addition of gas at the nozzle exitled to choking of the nozzle.

Nozzle-Jetting of Slag

Slag from raw iron production is melted at a temperature of 1400° C. Theliquid material is mixed with air at a ration of 0.7 Kg of air/Kg ofslag and the mixture is then nozzle jetted through a Laval-nozzle,whereby at the nozzle exit once again 0.7 Kg of air/Kg of slag wasadded. Powder of average grain size of d₅₀: 150 μm was obtained.

Obviously this invention is not restricted to the exact design or thelisted or described design examples, but various changes are possiblewithout deviating from the core scope and protection scope of theinvention.

1. A method for producing metal particles from molten metal comprising:mixing in a chamber molten metal with gas in a predefined ratio toobtain a hot gas/melt mixture having a ratio of gas to melt of 0.5-15kg/kg, wherein the chamber is provided with an outlet nozzle for the hotgas/melt mixture; and ejecting the hot gas/metal melt mixture bypressure through the outlet nozzle while adding further gas underpressure at the outlet nozzle to the hot gas/melt mixture leading to aparticle velocity of 50-100 m/sec to obtain fine liquid droplets whichsolidify while cooling to form particles of a controlled sizedistribution and shape, whereas the size distribution is controlled byvariation of the gas/melt ratio.
 2. The method according to claim 1,wherein the hot gas/melt mixture expands from the outlet nozzle of themixing chamber with a velocity increase of 10-100 times the originalconveying velocity of the hot gas/melt mixture to the outlet nozzle. 3.The method according to claim 2, wherein the expansion is carried outinto a chamber having lower temperature than the hot gas/melt mixture.4. The method according to claim 1, wherein the hot gas/melt mixture hasa ratio of gas to melt of 0.5-3 kg/kg.
 5. The method according to claim1, wherein the hot gas/melt mixture has a ratio of gas to melt of0.5-1.5 kg/kg.
 6. The method according to claim 1, wherein the metal isselected from the group consisting of: Zn, Ni, Al, Ag, Mg, Si, Ca, Cu,Mo, Pb, Ti, Sn, Li, Be, W, Fe, Co, Cr, Mn and alloys thereof.
 7. Themethod according to claim 1, wherein the gas is selected from the groupconsisting of inert gases, helium, argon, nitrogen, air, carbon dioxide,carbon monoxide, water vapor, combustion gas and mixtures thereof.